Principal point selector



Nov. 2, 1943.

H. w. WHITLOCK PRINCIPAL POINT SELECTOR Filed March 24, 1943 H W.WHITLOGK Patented Nov. 2, 1943 7 *Y' WQ P W INT E E t a HowardW. Whitlocli; San Diego; cans,- assignor" to Gl'aude imiwiokard, as 5 Secretary of Agrloul tureof: thet'Unitedr oflice; K

ApplicationMart'lr 24, '1 943; scram. 4811324 States, and: his: SIIGCBSSDI'S-Lillil mam, (c1. sa -18s harem-em undertheact ofivrareli 3,1883; as Y fl'llhistapplication'. is; mate 1- under the; act. on I March; 3, 1883; as; amended? by. the: act of Aprils.

30;. 19.28;. and: the invention herein described; if. patented, may: be: manufacturedi and: used:v by' or; fonfth' Government. ofctha United: States ofl America for governmental purposes without; the: payment lib! me: of; any royalty thereon;

- My inventionzhrelatess to: the compilation eof; maps by: photogrammetric: methods from" aerial? photographs, and:v its; object; is to provide: an: ins) strumentrfor u'sein selecting and: marking: on" aeriali photographs v1 the principal: points required.

by-sucmmethods. l x i principal. point selector embodying. my Iint-t vent-iohwis'iillustrated by theraccoinpanying draws" ing,uin:which-,. .t

- Figure: 1. is: a perspective? View; 'andl:

Figureirizr a partial" vertical section; showing.

As illustrated, my invention comprises a top cross-arm 8, a bottom cross-arm H! and a center assembly.

The cross-arms 8 and Ill are preferably made of a transparent plastic material; Each crossarm is uniform in breadth and. thickness throughout its length, the end portions being straight and operating in the same plane and the central portions being shaped to provide a The raised section of top cross-arm 8 has a height sufficient to allow it to.

raised level section.

overlie the raised section of bottom cross-arm III. 'A longitudinal centerline 9 is etched on the bottom surface of the end portions of each crossarm, a round opening is centered in the raised section of bottom cross-arm l0, and top crossarm 8 has an elongated slot 80. centrally located lengthwise of its raised section and is also provided at each end with a fastening prick 8b threads at the upper end of the collar 6 and are provided with a bearing surface by the washer 7, holdthe collar 6 in place while permitting movement of the cross-arms 8 and II],

t as required in the operation of the instrument.

The central shaft 3 is hollow for the greater part of its length, thus forming the i'nkreser- 'voir 322. Internal screw threads at the upper Theend:- of this hollow space-1 receive 'the screw 1 which. acts as a: coverfor the reservoir-325 the? top lock. nut 53 holds the central shaw -3 yieldingly in an upward position. The tendency ot the spring. ll to force the central shaf't 3 out: of the collar 6. is: restrained by a-flange at" the lower end of? the central shaft: it Below: this flange the centrail shaft: is threaded to r'eoei'vw t an inking device" Htiwhich- 113.518. central opening:

andza. concentric annular insert;v i lb: or ink=absorbing materiall. Qpenings': 3c and. Isl althrough the central. shaft 32 andlirrking device I lhrespe'ce. tivelt i provide: channelsvthrougmwhicmink: is: supplied from the ink reservoir 3b to the inkabsorbing insert Hb. A needlepoint 3d on the lower end of the central shaft 3 projects through a central opening in the inking device II and extends downward slightly below the level of the ink-absorbing insert III).

In the assembled arrangement of the instrument, the top cross-arm 8is placed across the bottom cross arm l0, and the center assembly passes through the elongated slot 8a in the top cross-arm 8 and fits in the round opening Illa in the bottom cross-arm H3. The elongated slot 8a and the opening [0a fix the position of the center assembly so that the needlepoint 3d is always located at the point of intersection of the centerlines 9 of the cross-arms 8 and Ill. The elongated slot 80, also makes possible both limited rotation and,lateral adjustment of the bottom cross-arm I0 after the top cross-arm 8 has been fixed in place by the fastening pricks 8b.

The function performed by my principal point selector is a. preliminary step in the compilation of maps from aerial photographs by photogram metric methods. Normally this step is accomplished by drawing axial connecting lines between collimation marks located at the four edges on each aerial photograph, piercing the intersection of these lines, which is the principal point, with a pointed instrument, and then marking the principal point by circling it in ink with a. bow compass. performed much more rapidly by lining up the centerline 9 of the top cross-arm 8 with one pair of collimation marks and pressing down slightly so that the fastening pricks 81) hold it in place, bringing the centerline 9 of the bottom cross- With my instrument this step can be aerial photograph so that an identifying circle is marked around the principal point.

A principal point must be selected and marked for each aerial photograph used in the compila-- tion of a map by photogrammetric methods. As

the number of aerial photographs for any one map may be quite large, the step is an often repeated one. The facility with which the step may be performed with my principal point selector results in a considerable saving in time.

surface, a transparent section centrally located lengthwise of the end portion of each cross-arm, reference lines on said transparent section delineating the longitudinal centerlines of the cross-arms, a pointed element located at the intersection of the longitudinal centerlines of the cross-arms, means for securing the pointed ele- ,ment on the cross-arms, means for vertically moving the pointed element in and out of con- 10" tact 'with the plane surface, yieldable means urging said pointed element upward out of con- 1 tact with the plane surface, and means for plac- King an identifying mark on the plane surface Having thus described my invention, I claim:.

1. An instrument of the character described comprising a cross-arm adapted at its end portions to rest on a plane surface and having a raised section midway of its length, a second cross-arm extending transversely at the raised section of the first named'cross-arm and adapted at its end portions to rest on the same plane surface, a longitudinal centerline on the end por tions of each cross-arm, :means rendering the central longitudinal section of each cross-arm transparent, a pointed element located at the intersection of the longitudinal centerlines of the cross-arms, and means for placing an identifying mark on the plane surface adjacent to points indicated by said pointed element,

2. An instrument of; the character described comprising a cross-arm adapted at its end portions to reston a plane surface and having a raised section midway of its length a second cross-arm extending transversely at the raised section of thefirst named cross-arm and adapted at'its end portions to rest-0n the same plane adjacent to points indicated by said pointed element. V

' 3. An'instrument of the character described comprising a transparent cross-arm adapted at its end portions to rest on a plane surface and having a raised section midway of its length,

a second transparent cross-arm extending transversely at the raised section of the first namedcross-arm and adapted at its end portions to rest on the same plane surface, means to prevent movement of the second named cross-arm relative to the plane surface, means for limited rotation and lateral adjustment of the first named cross-arm, a longitudinal c-enterline on the bottom surface of the end portions of each cross-arm, a pointed element located at the in tersection of the longitudinal centerlines of the cross-arms, meansfor securing the pointed element on the cross-arms, means for vertically moving the pointed element in and out of contact with v the plane surface, yieldable means urging said pointed element upward'out of contact with the plane surface, and means for marking a circle in ink on the plane surface around points indicated by'the pointed element.

HOWARD W. WI-IITLOCK. 

